In an internal combustion engine under a predetermined warm-up condition, the air-fuel ratio is enriched, and unburned fuel is discharged to an exhaust passage and is burned by the supply of secondary air. This increases the exhaust-gas temperature, and promotes activation of an exhaust-gas purification catalyst.
The supply of secondary air is controlled by a secondary-air valve provided in a secondary-air passage extending from a secondary-air supply source. A related art system discloses a temperature-sensitive check valve provided in a secondary-air passage downstream from a secondary-air valve so as to prevent exhaust gas from flowing into the secondary-air valve during normal operation.
A secondary-air valve is used under a predetermined warm-up condition, and is kept close during normal operation. When the exhaust-gas flow rate increases, the secondary-air valve is exposed to high-temperature and high-pressure exhaust gas due to backflow of exhaust gas. As such, the secondary-air valve can stick due to adhesion of combustion residues and the like contained in the exhaust gas, and parts can degrade due to heat.
By providing the check valve downstream from the secondary-air valve, as in the related art, the exhaust gas can be prevented from flowing back into the secondary-air valve. However, since the check valve is located in a thermally stricter condition (at a position near the exhaust system), the danger of system failure resulting from the check valve sticking is increased. In other words, if sticking occurs in the check valve, then secondary air is not supplied when needed, which reduces exhaust-gas purification performance.
An object of the invention is to more reliably prevent sticking and thermal degradation of a secondary-air valve.
According to an aspect of the present invention, a secondary-air supply source is driven and a secondary-air valve is opened under a high-load operating condition where the flow rate of exhaust gas is high, unlike a warm-up condition.
According to the present invention, under a high-load operating condition where the flow rate of exhaust gas is high, the secondary-air supply source is driven and the secondary-air valve is opened so that secondary air flows. As a result, exhaust gas that would otherwise flow back to the secondary-air valve is stopped, and combustion residues are prevented from adhering to the secondary-air valve. Moreover, the secondary-air valve is cooled and thermal degradation is prevented.